Circuits for discharge tubes



Oct; 31, 1939. c. J. R; H. ,VON WEDEL 2,178,011

CIRCUITS FOR DISCHARGE TUBES Original Filed April 6, 1931 INVENTOR (MU/{H102 M'eael ATTORNEYS Patented Oct. 31, 1939 UNITED STATES omomrs roa mscmmca "runes Carl R. H. yon Wedel, Newark, N, J., asslgnor, by mesne assignments, to General Electric Company, a corporation of New York Application April s, 1931, Serial No. 527,930 Renewed July 27, 1939 2 Claims.

This invention relates to circuits for gaseous discharge tubes. An object of the invention is to provide a system wherein the tube will be operated at maximum emciency and uniformity,

and to that end a minimum of'energy will be expended in heating the cathode and the proper control of the cathode will not interfere with the efliciency and uniformity of the discharge current.

The invention will be described with reference to the accompanying drawing, in which the figure diagrammatically represents apparatus embodying the invention. Referring to the drawing, l represents a discharge tube of the type commonly employed for illumination provided with incandescent cathodes and contains a gaseous medium such as neon, argon, mercury or the like, or a mixture of these or other suitable gases. The cathode elements 2, 3 are shown as suitable thimbles or cylinders coated with an electron emissive coating. I prefer a coating composed of an alkaline earth oxide combined with one or more of the oxides of metals capable of forming an amphoteric oxide less acid than titanic acid, such for example, as aluminum oxide, zirconium oxide, etc., whereby the combination being heated in a vacuum tends to form an emissive coating possessing high resistance to ionic bombardment or other causes tending to disintegrate the emissive coating. Inside of the cathodes 2 and 3 are heater elements 4 and 5 of suitable resistance material.

Because these elements are to be operated at high voltage several times exceeding the ionization potential of the gas used in the tube, particular arrangements must be made to avoid crossarcing along this heater wire and between this element and the cathode thimble. I prefer to use an arrangement whereby the heater wire is wound in a small diameter spiral which is wound helically on an insulating support, such as steatite or soapstone, provided with helical grooves to receive the heater spiral and whereby the space between the thimble and said support is filled with an insulating powder such as mixtures of zirconium oxide, beryllium oxide or other heat.

resistive non-conducting material, which prevents the development of a gaseous discharge within the thimble. Such cathodes are described and claimed in my co-pending application Serial No. 243,042, filed December 28, 1927.

The heating elements 4 and 5 are connected to the cathodes preferably as indicated in the drawing, and connected together by a conductor 6 in 56 which may be placed an adjustable resistance 1.

The heater elements in series are connected to the opposite poles of a source of alternating current ill by conductors 8 and 9 respectively. By reason of the high voltage employedfor the heating elements 4 and 5 the current required for this 5 purpose will be relatively small.

In conductor 9 is an impedance ll, preferably an induction coil, and in conductor 8 is an impedance [2, preferably an adjustable resistance. These impedances are of such value that under 10 the normal heating current load of the tube they ofier substantially no opposition to the flow of current, but upon a. rise of the main discharge current to normal load, will increase in value as the current in the circuit increases. The cathodes 2 and 3 are surrounded by cylindrical auxiliary electrodes l3, H, the auxiliary electrode l3 being connected to the cathode 2 by means of conductors l5 and 8, and the auxiliary electrode l4 being connected to the cathode 3 by means of 20 conductors l6 and 9.

In the conductors l5 and I6 are located impedances I! and I8 respectively, which impedances are so proportioned as to be low enough to permit an earlystart of the auxiliary discharge, 25 which introduces the main discharge on the ascending slope of the rising supply voltage. On the other hand, the resistances l1, 18 must be sufliciently large to avoid an undesired loss of energy in the auxiliary discharge circuit.

The auxiliary electrodes l3, l4 should be separated from the cathode by a distance greater than the mean free path of the electrons at the gas pressure used in the tube. I prefer to construct these auxiliary electrodes in the form of cylin- 35 ders surrounding the main electrodes, so that they serve as shields or reflectors with regard to the heat radiated from the main electrodes, thus decreasing the loss in heating current. v

In the operation of the tube the generator, 40 which preferably is a commercial lighting circuit at say 120 volts, will supply the heating energy to the cathodes, such energy being limited or finally regulated by the adjustable resistance 1, if necessary. 7

At the same time that the cathode thimbles heat up to emitting temperature the auxiliary circuits will be energized. For example, the auxiliary circuit for the auxiliary electrode I 4 will be from generator Ill through the inductance ll, conductor l6, impedance l8, auxiliary electrode l4, cathode 2, conductor 8, impedance l2 to generator. When the voltage and current in this circuit rises to a value suflicient to ionize the space between. the auxiliary electrode and the cathode, the main discharge will start between electrodes 2 and 3 and the cathode would thereby be heated excessively. When the main discharge commences, however, the current through the impedances H, i2 will increase and be added to that in the auxiliary circuits, and this increased current value in the impedances will cause a proportional voltage drop across the heater elements and a reduction of the heating effect thereof on the main electrodes. The main discharge path operates thereby as a voltage regulating shunt for the heating current and the starting auxiliary discharge circuits combined. This has the following advantages: The tube will heat up quickly, because so long as the main electrodes are not suificiently emissive, almost the total supply voltage is available across the heater elements. The moment, however, that the auxiliary and main discharges commence, the heating effect of the heater elements is proportionally reduced, as the discharge current in the tube is permitted to increase, so that the temperature of the main electrodes is maintained within the desired operating limits.

As an example of the operation in a practical device, agenerator voltage of 120 may be assumed, the tube geing adapted to carry a maximum load of 2% amperes between the main discharge electrodes. The cathodes may be designed to carry a heating current of 0.2 ampere of current at 55 volts, so that the normal heating energy should be about 10 watts to heat the cathodes at a normal temperature of about 850 centigrade before the main discharge starts. The energy required for the heating will be reduced by reason of the surrounding metallic auxiliary electrodes which tend to prevent radiation of the heat.

When the main discharge starts the normal operating voltage across the main electrodes will be forexample about '70 volts. Therefore the impedances H, H should be of such order as to cause a drop of about 50 volts in the line voltage under full load conditions of the tube. A convenient arrangement is to make the impedance I l cause a drop of about 48 volts and use as impedance I! a resistance adjusted according to the total desired discharge current.

Before the main discharge commences the impedances H and I2 will cause relatively low or negligible drop at the 200 milliamperes current consumed by the heating coils of the cathode. When the coating of the cathode becomes hot enough to emit electrons, an electron current will flow between the cathode and adjacent auxiliary electrode, and this, as before stated, will start the auxiliary gaseous discharge at an instant when the voltage across the main electrodes is comparable to the operating voltage across the main discharge path, and almost instantly the main discharge also, thus causing a heavy current of 2 amperes to flow, which heats the electrodes 2 and 3 by bombardment, and also by heat radiation of the ionized gas. At this time, however, the voltage across the heater elements is prevented from increasing above '70 volts so that the effective current of the heater elements is reduced correspondingly and also the voltage between the starting electrodes and the main electrodes, thereby preventing unnecessary waste of current either for heating or starting purposes and disintegration of the cathode.

I claim:

1. In a, gaseous discharge system a discharge tube having indirectly heated electron emissive cathodes, the heating elements for the cathodes being adapted for a voltage higher than the ionization potential of the gas employed in the tube. and auxiliary starting electrodes, the main discharge path connected in shunt to the circuits of the auxiliary electrode discharge circuit and the heating elements, and means whereby upon the starting of the main discharge the current in the heating circuit and the current in the auxiliary discharge circuit will be reduced.

2. In a gaseous discharge lamp system, the combination of a tube having an indirectly heated electron emissive cathode and a surrounding auxiliary electrode, an anode, a heater circuit and an auxiliary discharge circuit, and impedances in the several circuits so related to each other that upon the establishment of the main discharge, the current in the heater circuit and in the auxiliary discharge circuit will automatically be reduced by the main discharge path operating as a voltage regulator in shunt thereto.

CARL J. R. H. VON WEDEL. 

